Micro-Electro-Mechanical Systems (MEMS) is a widely used technology that enables integration of both microelectronic circuits and mechanical structures on the same chip, while significantly lowering fabrication costs and chip size. Oftentimes, a surface micromachining technique is utilized to fabricate MEMS devices, wherein structural parts of the device are combined with layers of a sacrificial material. The sacrificial material is then removed by employing a chemical etchant that does not react with the structural material, leaving behind movable structural parts. Typically, MEMS processes using sacrificial oxides are well suited to produce fragile structures such as thin membranes or low stiffness mechanical devices by mechanically supporting such devices during the fabrication process and releasing the sacrificial layers as a last step in the process. The most widely used surface micromachining techniques use Silicon dioxide (SiO2) as the sacrificial material Hydrofluoric acid (HF) as the chemical etchant.
For MEMS structures that are integrated with a CMOS wafer, the CMOS wafer is susceptible to damage due to the exposure to the hydrofluoric acid (HF) based chemical etchant during MEMS release. Moreover, a conventional CMOS wafer contains silicon oxide in its inter-metal dielectrics and passivation stack, which etches rapidly on exposure to HF. Specifically, passivation openings on the CMOS wafer that are utilized to expose metal pads for bonding the CMOS wafer to the MEMS structure, expose the wafer's silicon oxide, making the wafer unsuitable for HF exposure during fabrication of integrated CMOS-MEMS devices.